1. Field of the Disclosure
The present disclosure relates generally to geological exploration in wellbores. More particularly, the present disclosure describes an apparatus, a machine-readable medium, and a method useful for obtaining improved measurements of formation pore pressure downhole.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. During drilling operations, an important aspect of drilling operations is to measure the formation pore pressure. Numerous methods exist for measurement of formation pressure. For example, U.S. Pat. No. 7,266,983 to Krueger et al., having the same assignee as the present disclosure, teaches a Measurement While Drilling (MWD) method of determining a formation pressure during drawdown of a formation comprising sampling fluid from a formation using a downhole tool. A fluid sample pressure is determined at two different times during the drawdown. The fluid sample pressures are analyzed using a higher-order pressure derivative with respect to time technique to determine the formation pressure during the drawdown. Another method of determining a formation pressure during drawdown of a formation comprises sampling fluid from a formation using a downhole tool. A fluid sample pressure is determined at two different times during the drawdown. The fluid sample pressures are analyzed using at least two analysis techniques to each determine an estimate of the formation pressure during the drawdown. The measurement of formation pore-pressure is important in the implementation of a completion scheme for a production borehole and further development of a reservoir. Wireline methods also exist for estimation of formation pore pressure. See, for example, U.S. Pat. No. 5,473,939 to Leder et al.
Once a borehole has been cased, measurement of formation pore pressure from downhole measurements becomes more complicated. In prior art methods, it is necessary to perforate the casing to make such measurements or, in the case of producing wells, disconnect the production tubing to carry out the pressure tests. Additional testing of formation pressure in a cased hole is necessary in at least three situations. The first situation is to measure the formation pore pressure in a producing reservoir. This may be needed, for example, to alter injection operations in a secondary recovery operation. The method of the present disclosure enables the pore pressure measurement to be done without having to shut down production to conduct time-consuming draw-down or pressure buildup tests. A second situation arises when measuring pore pressure in a cased hole is to select additional intervals for conducting perforation operations to develop previously bypassed oil and gas. A third reason for measuring formation pore pressure is to serve as a check on completion operations: there are instances in which perforations may be partially blocked, and a measurement of formation pore pressure may identify when this has occurred.
The present disclosure is directed towards an apparatus and method for measuring formation pore pressure through casing without lengthy draw-down or pressure buildup tests.